In the U.S. Pacific Northwest, a large, seasonal labor-force is required for the production of tree fruit crops like fresh market apples, cherries, and pears. The most time and labor-intensive task in fruit crop production is harvesting. In Washington State alone the apple and pear harvest requires the employment of approximately 30,000 additional workers with an estimated harvest cost of $1,100 to $2,100 USD per acre per year. To reduce harvesting costs and dependence on seasonal labor, researchers have developed shake-and-catch systems for the mass harvesting of fruits such as berries, cherries, and citrus. These techniques, which apply vibration to the trunk or branch of the tree in order to separate the fruit, are typically used to harvest fruit destined for the processing market where there are established tolerances for fruit bruising and external defects. There have been some attempts to develop mass harvesting systems for fresh market citrus, cherries, and apples, but the systems demonstrated marginal rates of fruit detachment, were only efficient with compatible tree-training systems, or frequently harvested fruit without stems.
The use of robotics technology is another approach researchers have tried for the harvesting of tree fruit. For economic reasons related to changing labor conditions, scientists and engineers started to actively work on research and development of fruit-picking robots in the 1980s. These earlier research efforts defined the basic functional requirements of a fruit-picking robot as the following: i) locate the fruit on the tree in 3D dimensions; ii) approach and reach for the fruit; and iii) detach an undamaged fruit from the tree and deposit it in a container. In order for a fruit-picking robotic system to be commercially viable, it has to be economically feasible and provide harvesting rates (e.g. fruit/second) comparable to those obtained through manual harvesting. Additionally, the system should minimize damage to both the plant and the harvested fruit to a tolerable level. Despite numerous attempts to transfer industrial robotic technology directly to field based, biologically driven environments, the mechanization of specialty crop harvesting has achieved only limited success primarily due to inadequate accuracy, speed, and robustness.
Fruit in a single crop possess a high level of variability. For example, tree fruit vary in position, shape, size, and growing orientation. Even for the same apple cultivar, parameters such as size and stem length vary widely within a single tree. There also exists a year-to-year variability in these parameters. Fruit removal technique is usually the largest cause of fruit injury. Insufficient automated devices exist for fruit harvesting which are able to accommodate these requirements.
Because of rising labor costs, a high workplace injury rate due to ladder use, and increasing uncertainty about the availability of farm labor, the lack of mechanical harvesting is a critical problem receiving much attention from both federal agencies (e.g., United States Department of Agriculture) and state and local organizations (e.g., Washington Tree Fruit Research Commission).
The basic functional requirements of an apple picking end-effector are to approach and reach for the fruit and then detach an unblemished apple from the tree. In addition to being efficient, productive and economically feasible, it is important that the system not damage the picked fruit, adjacent fruit, or the tree. The end-effector can damage the apple by applying excessive force during picking or by employing inappropriate stem separation techniques. Some different techniques have been investigated for end-effector designs.
Bulanon and Kataoka (Bulanon & Kataoka, 2010) designed an end-effector that used a peduncle holder to apply pressure against the peduncle before removing the fruit with a lifting and twisting motion. Though this technique minimized damage to the fruit, the system was constrained in that the end-effector had to approach the apple horizontally.
Baeten et al. (Baeten, Donne, Boedrij, Beckers, & Claesen, 2008) developed a novel gripper consisting of a flexible silicon funnel that used vacuum suction to activate the gripping function. During field tests the average harvesting time was approximately nine seconds, but stem pulls occurred with approximately 30% of the harvested apples. It was also important to sequence apple selection so that adjoining apples in a cluster would not interfere with the picking process.
Zhao et al. (Zhao, Lu, Ji, Zhang, & Chen, 2011) proposed a cutting end-effector utilizing multiple sensors that demonstrated impressive fruit detachment rates during field tests. Although cutting minimizes the likelihood of stem pulls, it usually requires more complex control requirements, which can lead to higher costs.